We hear from scientists and health officers that we should all get at least half an hour of exercise a day to stay healthy. And if you already do that you should try to make that half hour a whole hour. This guideline is on the low side, epidemiologists at the University of Washington report in the BMJ. According to their meta-analysis, our health only really improves if we get two to three hours of physical activity a day.

Study
The researchers accessed 174 epidemiological studies on the relationship between physical exercise and the risk of chronic disease. They then gathered all the data and reanalysed it. The researchers confined themselves to ischaemic heart disease [heart attacks in plain language], strokes, breast cancer, bowel cancer and type-2 diabetes.

Results
For most of the chronic diseases that the researchers investigated, the relationship between physical exercise and the risk of contracting the disease looks like the one shown in the figure below. The one illustrated is for heart attacks.

The more exercise people get, the lower the chance of a heart attack. The most protective effect of exercise is gained by doing between 3000 and 4000 Met minutes of physical activity. Doing more exercise than that will not have a bad effect, but is unlikely to add any health benefit.

The figure below shows the effect of physical activity on the chance of heart attack, stroke, breast cancer, bowel cancer and type-2 diabetes.

Conclusion
At present the World Health Organization advises at least 600 Met minutes of physical activity per week. That's the equivalent of 150 minutes of brisk walking or 75 minutes of running.

In rich countries information officers translate that into 30 minutes of activity a day. They assume that inhabitants of those countries are so spoiled that on at least two days a week they do absolutely nothing, not even walking a small distance.

The study published recently in the BMJ suggests that positive health effects will only be gained by doing much more than 600 Met minutes of exercise – somewhere between 3000 and 4000 Met minutes to be exact. So that's seven times more than that half an hour of walking.

"A person can achieve 3000 metabolic equivalent minutes per week by incorporating different types of physical activity into the daily routine", explained first author Hmwe Kyu in an interview with The Guardian, an English newspaper. [theguardian.com 9 August 2016]

"For example, climbing stairs for 10 minutes, vacuuming for 15 minutes, gardening for 20 minutes, running for 20 minutes, and walking or cycling for transportation for 25 minutes, on a daily basis, would together achieve about 3,000 metabolic equivalent minutes a week."

In practice only athletes and people who do physical work are likely to attain the optimal amount of physical activity.